Add (temporary) MIR builders.

[eddyb]

Until MIR is turned on by default for everyone, we want to make sure we don't regress.

In rust-lang/rust#32080 I've tagged relevant functions in tests that involve features not yet in MIR (such as debuginfo or overflow checks) with #[rustc_no_mir], and we can keep adding those as long as we track them, so I believe the new MIR builders should gate all builds.

We're going to need at least one UNIX builder and one MSVC because of the unwinding complexities found in the latter (I don't even know if MSVC can bootstrap w/ MIR right now).

Blocked on the snapshots being added in rust-lang/rust#32345. cc @alexcrichton

[alexcrichton]

This is just enabled with ./configure --enable-orbit, right?

[alexcrichton]

And then we just run make check?

[eddyb]

@alexcrichton Correct, I did this on a x64 linux machine and it completed successfully.

[alexcrichton]

Ok, done in 4d3bc18, will deploy shortly